The present invention is concerned with the preparation of faujasite type zeolites in microcrystalline form.
Faujasite is a fairly rare naturally occurring aluminosilicate mineral of defined crystal structure. Products having substantially the same crystal pattern as this natural mineral have been synthesized by reaction of silicon compounds with aluminum compounds in aqueous alkaline solutions. Among the synthetic faujasite products thus synthesized which have found extensive industrial use are so-called X-type zeolites and Y-type zeolites. The preparation of X-type zeolites is described in U.S. Pat. No. 2,882,244. The preparation of Y-type zeolites is described in U.S. Pat. No. 3,130,007. These synthetic faujasites may be represented by the general formula: EQU Alk.sub.2 O:Al.sub.2 O.sub.3 :2-7SiO.sub.2 :nH.sub.2 O.
By the usual methods of the prior art, these synthetic faujasites are produced in the form of crystals having an average size range of 1 to 3 microns. It is proposed in U.S. Pat. No. 3,516,786 to produce synthetic faujasites in microcrystalline form, stated to be in the 100-1000 Angstrom range. In the method disclosed in said patent, an alkali metal silicate solution is admixed with an alkali metal aluminate solution in proportions to provide a SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of from 4 to 20 and the obtained reaction mixture is aged for 6 to 24 hours at moderate temperature; the aged composition is then heated for 2 to 16 hours to effect crystallization in the presence of a water-miscible organic solvent added to the composition at any stage prior to the heating step.
Because of the relative volatility of the organic solvents proposed in accordance with said prior patent, part of this additive is driven off during the crystallization of the product at elevated temperature, so that the concentration of such additive is not constant. This disadvantage is overcome in the crystallization of the faujasite reaction product by the additives employed in the present invention.